EVOLUTION OF POROSITY AND TEXTURE INTHERMAL BARRIER COATINGS GROWN BY EB-PVD

Scott G. Terry, Jennifer R. Litty and Carlos G. Levi

Materials Department., University of CaliforniaSanta Barbara, CA, 93106-5050

A b s t r a c t

The pattern and distribution of porosity in the columnar microstructure of thermal barriercoatings (TBCs) grown by electron-beam physical vapor deposition (EB-PVD) are key factors indetermining the coating compliance, and consequently its resistance to spallation, as well as itsthermal conductivity, and hence the requisite thickness for a given degree of insulation. The pre-sent study aims to advance the understanding of the evolution of porosity during EB-PVDgrowth, as well as its relationship with the concurrent evolution of crystallographic texture.TBCs with the conventional 7 wt.% yttria partially-stabilized zirconia composition were depos-ited on stationary substrates at temperatures of 900°C (~0.40TM) and 1100°C (~0.46TM). Thesubstrates were shaped to explore deposition under normal and oblique (45°) vapor incidence inorder to provide insight on the shadowing mechanisms responsible for the formation of porosity.It was shown that the characteristics of the porosity can change dramatically with both vapor in-cidence angle (VIA) and deposition temperature. The column growth direction was found to be⟨101⟩ for both normal and oblique incidences at 1100°C, but changed from ⟨111⟩ to predomi-nantly ⟨101⟩ for deposition at 900°C. The role of substrate manipulation during deposition is dis-cussed in the context of these findings.

Elevated Temperature Coatings: Science and Technology IIIedited by J.M. Hampikian and N.B. Dahotre

The Minerals, Metals and Materials Society, Warrendale, PA, 1999, pp. 13-26.

I n t r o d u c t i o n

Thermal barrier coatings (TBCs) have emerged as arguably the most critical materials issuefor the next generation of gas turbine technology [1]. The addition of TBCs to turbine airfoils isestimated to have the potential for increasing their temperature capability by as much as ~150°C, aperformance improvement equivalent to that produced by the last 20 years of alloy developmentand cooling engineering [2]. Notwithstanding a record of nearly three decades of service in gasturbines, full realization of the TBC potential remains hindered by concerns about their reliabilityand a lagging science base that can guide their optimization.

In addition to the obvious requirements of refractoriness and low thermal conductivity,TBCs must have “strain tolerance”, i.e. high resistance to spalling under thermal cycling [3].The latter is not a property of the thermal barrier alone but rather of the complete material system,as the stresses that drive spalling arise primarily from the mismatch in coefficients of thermal ex-pansion (CTE) between the substrate, the TBC, and the thermally grown oxide (TGO) whichforms between the TBC and the underlying “bond coat” (BC) [2]. From this perspective, theideal TBC should have a high in-plane compliance in order to minimize its contribution to the re-sidual stresses in the coating. (The stresses are then dominated by the TGO/substrate mismatch,whereupon TBC life could be conceptually related to the attainment of a critical TGO thickness,in qualitative agreement with practical observations.) This argument has been used to explainwhy columnar TBCs produced by electron-beam physical vapor deposition (EB-PVD) are gener-ally more durable than those produced by plasma spray (PS) [3].

Current TBCs are nearly universally based on ZrO2 partially stabilized with ~7wt.%Y2O3(YPSZ) [4]. The coatings tend to exhibit significantly lower thermal conductivity and highercompliance in comparison with the monolithic oxide alone, revealing important contributionsfrom the microstructure. Perhaps the most critical feature in this regard is the porosity and itsassociated parameters, i.e. concentration, morphology, size scale and spatial distribution. Forexample, both EB-PVD and PS coatings can be viewed as containing relatively large voids ofhigh aspect ratio. However, the ribbon-like voids in the former are aligned normal to the plane ofthe coating, separating the columnar grains and yielding superior compliance [3], whereas thedisk-like voids in the latter occur between the splats, parallel to the plane of the coating and pro-moting superior insulating efficiency [2]. Recent work has further suggested that the graded mi-crostructure inherent in EB-PVD TBCs, where the coating density (and consequently the in-planemodulus) decrease with distance away from the substrate [3], could have major implications inthe mode of failure and the strain tolerance of the TBC [5]. EB-PVD coatings also contain amuch finer scale of porosity within the columns [6] which can have, in principle, a greater effecton the thermal conductivity of the coating than the intercolumnar voids.

Notwithstanding the obvious importance of porosity to the thermal, mechanical, and evenchemical performance of the TBCs, surprisingly little research has focused on quantifying its re-lationship with the fundamental coating properties, or on establishing and quantifying its depend-ence on processing conditions. An effort in the latter direction has been initiated by the authorsfor coatings produced by EB-PVD. This paper reports on the effects of temperature and vaporincidence angle (VIA) on the evolution of intercolumnar porosity in TBCs, as well as on the re-lated evolution of crystallographic texture.

A key issue to be examined is the apparent inconsistency between the purported microstruc-ture and compliance goals for the TBCs, and the temperatures normally used in their deposition.In the context of the conventional “structure map” (Figure 1), TBCs are rather unique becausethey aim to combine the well aligned columnar structure of Zone 2 with the substantial porosity

Figure 1. Schematic structure map, adapted from Reference [7]. The transition temperatures betweenzones were reported as ~380°C (1-2) and ~1000°C (2-3) for YPSZ deposited at 20-40nm/s.

characteristic of Zone 1. This would place the desirable microstructure at the transition betweenZones 1 and 2, which for ZrO2 is reported at ~380°C (~0.22TM) [7]. However, TBC depositionis usually performed above ~1000°C (~0.42TM) [8], which is the reported temperature for theZone 2-3 transition [7]. The high deposition temperatures are apparently selected to promotestrong bonding with the underlying BC [8], and indirectly as a means to enhance the stability ofthe coating against sintering during service. Porous columnar TBCs with desirable complianceare nonetheless obtained under these high temperatures, at variance with zone structure modelswhich would predict the microstructure to be dense and probably equiaxed (T/TM ~0.5). Booneet al. [9] first noted that intercolumnar gaps evolved in normally dense metallic films when thesubstrate was rotated during coating, an observation that led to the “segmented” microstructureconcept for strain tolerant TBCs [3]. Schulz et al. [10] ascribed the effect to “shadowing” andlater proposed a modification of the structure map to account for the effects of rotation [11]. Atthis point, however, there is only a superficial understanding of the relevant mechanisms in theTBC literature.

Because the intercolumnar pores evolve with the aligned columnar structure, elucidating theorigin of the former also implies understanding the mechanisms that give rise to the latter. Muchof the literature on thin films views the columnar structure as a result of the evolution of pipelikevoids produced by a combination of insufficient surface diffusion and atomic-scale shadowing.Columnar growth, however, is more often the consequence of “evolutionary selection” [12]wherein grains (within a random polycrystalline array) having their preferred growth directionsoriented in the direction of the vapor flux gradually occlude less favorably oriented neighbors.The result is a film with an out-of-plane fiber texture consistent with the preferred growth direc-tion. The literature, however, reports not one but several crystallographic textures in YPSZTBCs, notably ⟨100⟩ [6, 13-16] but also ⟨113⟩ [17-19] and ⟨111⟩ [19, 20]. The multiplicity oftextures has been ascribed to variations in the processing conditions but clear relationships havenot been established and some disagreement exists among different studies.

The strategy adopted to shed additional light on these issues involves establishing first abaseline for microstructure evolution in the absence of rotation. Particular attention has beenplaced on accurately measuring and controlling the substrate temperature during deposition, acritical parameter which is often uncertain in previous studies. The microscopic aspects of shad-owing have been explored by deposition on stationary substrates oriented at different angles rela-tive to the vapor flux.

DepositionRate Sensor

1” Dia x 6” longZirconia Ingot

Substrate Heater

Substrate

E-beam

Vacuum Chamber

270° E-gun and water-cooled hearth

CMD Pressure Gauge

Source Shutter

Oxygen Mass Flow Controller

O2

O2

ThermocoupleAC Power

Supply

(a)

15 cm

90°

6°11°

6 cm

VIA

(b)

Figure 2. Schematic of the UCSB EB-PVD system as used for the experiments in thispaper (a) and detail of the “V” specimens used to study oblique deposition (b).

Experimental Procedures

All deposition experiments were performed in house using the electron-beam PVD systemschematically depicted in Figure 2(a). The system consists of a cubic vacuum chamber, 60 cmon the side, with a 10 kW, 270° electron gun and a water-cooled copper hearth modified to accepta continuously fed ceramic ingot 25 mm in diameter. The gun is driven by a 10kV, 14kW powersupply (AIRCO/Temescal CV-14). The deposition rate is monitored using a quartz-crystal sen-sor (Leybold-Inficon CrystalSix), shuttered intermittently so it can operate effectively at highrates over long periods of time, whose signal is fed into a close-loop controller for the e-gun(Leybold-Inficon IC/5). Separate controls are available for the beam sweep pattern and rate(AIRCO VWS-1090). An electronic mass flow control system (MKS 1159A) meters a presetflow of oxygen into the chamber during deposition. Total chamber pressure is regulated via afeedback-controlled variable conductance exhaust valve using a capacitance manometer sensor.

The deposition rate in the present set-up is limited by the maximum total pressure that the270° gun can tolerate (~10-4 torr) before it turns itself off automatically to prevent rapid filamentdegradation. As the system is evacuated to 10-6 torr prior to evaporation, the total pressure isessentially comprised of the evaporated ceramic plus the excess oxygen fed into the chamber (at~20 sccm) to promote stoichiometric deposition. At present, deposition rates for stoichiometricfilms are limited to ~2 µm/min (referred to a stationary substrate under normal incidence), whichis significantly lower than current industrial practice but in the range used to develop the structuremap of Figure 1. The deposition rate can be readily increased up to ~4 µm/min with the same

oxygen feed rate and without exceeding the operating pressure limit, but the resulting coatingstend to be oxygen deficient (manifested by a grayish color). System modifications are underwayto extend the feasible range up to ~20 µm/min.

The ceramic ingots used in this investigation were 25 mm dia. x 150 mm long with nominalcomposition ZrO2-7wt.%Y2O3 (Trans-Tech, Adamston, MD). Major impurities reported are (inweight percent) 1.35HfO2, 0.08TiO2, 0.02SiO2, with ≤0.01 each of CaO, MgO, Al2O3, Fe2O3,Na2O, U and Th. The ingot density is 3.77Mg/m3, equivalent to a porosity content of ~38%.

The substrates were rectangular strips 70 x 10 x 1 mm cut from Fecralloy® sheet, nominallyFe-22Cr-4.8Al-0.3Si-0.3Y (in wt.%). 1 The sheet was vacuum annealed for 17 hours at 1100°Cand then mechanically sheared into strips, which were subsequently polished to a final media sizeof 3 µm. Some of these strips were hot formed into a “V” shape (cf. Figure 2b), using a bendingpress, and then hand polished with 3 µm diamond paste to remove the oxide generated during theforming operation. Flat and bent substrates were oxidized in air for 12 hours at 1100°C to form astable α-Al2O3 layer about 1.5 µm thick prior to deposition of the TBC. The substrates werethen mounted on a stage (Figure 2a) and resistively heated to the desired deposition temperatureusing 20V AC power. For the present experiments, the temperature was monitored using a TypeK thermocouple spot-welded to the substrate, and controlled within ±10°C of the desired value.

The deposition experiments performed for this study are summarized in Table I. The sub-strate temperatures are equivalent to ~0.4 and 0.46TM in the homologous scale, representing ap-proximately the bounds of the range typical of current industrial practice. The flat specimensprovide a baseline for normal deposition (0° VIA), whereas the “V” specimens are intended toexplore the effect of high vapor incidence angles (~45°VIA) on the TBC microstructure. The tipand “wing” sections of the “V” (Figure 2b) receive approximately normal fluxes and provide auseful comparison with the microstructures found in the flat substrates. The processing condi-tions were selected to give approximately the same rate of coating deposition normal to the sub-strate plane in all cases (~20 nm/s or 1.2 µm/min). This implied the use of higher vapor fluxesfor the “V” specimens, by a factor of cos(VIA)-1 (Table I), and hence faster evaporation rates.

The coatings were characterized primarily by X-ray diffractometry (XRD) and scanningelectron microscopy (SEM). Observations were made on both polished cross sections andgrowth surfaces, as well as fracture surfaces of some samples generated by notching the sub-strate with a diamond saw and breaking the specimen in bending. Cross sections were preparedby cutting sections of the coating and substrate with a low speed diamond saw, placing twopieces in a “sandwich” configuration with the coating in the center, impregnating the assemblagewith epoxy resin, and polishing using conventional metallographic techniques. Polishing mediawas diamond paste with colloidal silica for the finishing step.

Table I. Deposition conditions for TBCs studied in this investigation.

NominalVIA

SubstrateTemp. (°C)

Thickness(µm)

Depositiontime (min)

Average Rate(µm/min)

Rate Normalto Vapor Flux

0° 1100 120 96 1.3 1.3

0° 900 110 88 1.3 1.3

45° 1100 65 56 1.2 1.6

45° 900 61 62 1.0 1.4

1 Material supplied by Goodfellow, Berwyn, PA. The actual Y content is typically ~0.05 wt.%.

Figure 3. SEM cross sectional views of YPSZ coatingsdeposited under normal incidence (nominal VIA 0°).(a) 900°C, (b) 1100·C. The dark phase near the bot-tom is the TGO formed during the oxidation treat-ment prior to deposition.

R e s u l t s a n d D i s c u s s i o n

General views of the microstructure ofthe coatings are shown in Figures 3 to 5. Acomparison of Figures 3 and 4 reveals thatthe coatings produced under nominallynormal incidence (flat substrates located di-rectly above the source and normal to theingot axis) appear much denser than thoseproduced under oblique fluxes (“V” speci-mens) at equivalent temperatures. Thestructure is fully columnar in all cases, al-though this is more difficult to discern in the0° coatings because of a lack of clearly de-veloped intercolumnar gaps. Nevertheless,closer examination shows that there is in-deed a non-negligible amount of high-aspect-ratio porosity aligned in the growthdirection in these 0° specimens. These poresmay extend over significant fractions of thethickness, but often show contact pointsand in some cases appear to be composed ofarrays of very small circular voids. What isparticularly significant is that these 0° coat-ings depart from the structure map, eventhough they were deposited at rates compa-rable to those used by Movchan and Dem-chishin [7], at temperatures in the upper endof Zone 2 (cf. Figure 1), and without anyrotation that could promote shadowing. Apossible scenario to account for the evolu-tion of this porosity will be discussed later.

The cross sectional views in Figure 4reveal the dramatic effect of the vapor inci-dence angle in developing the intercolumnargaps, even in the absence of substraterotation. The effect is most striking at thehigher temperature, cf, Figs. 3(b) and 4(b),where the individual columns become dis-tinctly defined and separated by wide gapsas the flux changes from normal to oblique.The intercolumnar gaps originate very closeto the substrate and the evolutionary selec-tion process is clearly reflected in the coars-ening of their spacing with increasing thick-ness.

Figure 4. SEM cross sectional views of coatings deposited under oblique vapor incidence (nominal VIA45°). (a) 900°C, (b) 1100·C. The column orientation angle is ~31° in (a) and ~36° in (b). The scale of themicrographs is the same as in Figure 3 but the total thickness of the coatings is somewhat lower, asindicated in Table 1.

The coating grown under oblique incidence at the lower temperature, Figure 4(a), alsocontains well defined elongated voids that initiate near the substrate, but the column definition isless distinct suggesting that the columns themselves may have long intragranular gaps. The dif-ferent microstructural scale between Figures 4(a) and (b) is qualitatively consistent with the re-finement of the porosity that would be expected from the effect of the lower temperature on sur-face diffusion. A correspondingly higher porosity content should also be anticipated for thelower temperature, but this is not immediately evident from the micrographs and needs to be as-certained by more quantitative measurements.

The shadowing phenomena responsible for the evolution of this elongated porosity is re-vealed by the micrographs in Figure 5, showing details of the column tips on the top surface ofthe coatings corresponding to Figures 3 and 4. It is first noted that there is no significant evi-dence of open gaps intercepting the surface under normal incidence, Figs. 5(a) and (b), in agree-ment with the nearly dense appearance of the cross sections in Figure 3. In contrast, the growthsurfaces in Figs. 5(c) and (d) show well developed voids on the side of the columns where a“shadow” would be produced by the interplay between the column tip and the vapor flux. Closerexamination reveals that the downstream profile of these voids closely replicates the shape of theedge of the tip. It is also evident from Figs. 5(c) and (d) that the finer scale of the columnarpores observed at the lower deposition temperature, Figure 4(a), is a direct consequence of thefiner scale of the crystallographic tips casting the shadows that give origin to them. The linkbetween porosity and texture is also reflected in these observations, as the type and configurationof the facets that form the tip is directly related to the growth direction of the columns.

Figure 5. SEM views of the top (growth) surfaces for the specimens in Figures 3 and 4: (a) 900°C, VIA0°; (b) 1100°C, VIA 0°; (c) 900°C, VIA 45°; (d) 1100°C, VIA 45°. The vapor flux in (c) and (d) descendedfrom top to bottom at an angle of ~45° from the plane of the image. Note the clear connection betweenthe voids and the “shadows” cast by the column tips as they interact with the vapor flux.

Texture Evolution

Examination of Figure 5 from a crystallographic perspective reveals that the tips evolvingfrom deposition at the higher temperature have the appearance of “rooftops” with well developedfacets. Figure 5(b) suggests that these dominant facets grow by ledge propagation and shouldcorrespond to the slowest growing planes for ZrO2, with the preferred growth direction definedby the arrangement of these facets [21]. A Periodic Bond Chain (PBC) analysis for the fluoritestructure [22] predicted that the dominant facets should be of the {111} type. Neglecting theslight tetragonality of the t’ phase in these coatings [19], it can be readily shown that ⟨001⟩,⟨111⟩, and ⟨101⟩ growth directions for the columns can be defined by tips consisting of {111}facets arranged as, respectively, a four sided pyramid, a three sided pyramid, and a two-sided“rooftop”. Pole figure analysis of the 1100°C specimens has shown the column axes to be con-sistently of the ⟨101⟩ type for both 0° and 45° VIA [23]. The angle between the bounding facetsis ~110°, suggesting that they are of the{111} type in agreement with the PBC argument. Thecolumns of the 0° coating do not appear to have any preferred orientation in-plane but develop asecondary alignment mode as the VIA changes to 45°. This is manifested by an alignment of the“rooftops” in Figure 5(d), compared with a more random distribution in Figure 5(b).

The 900°C coatings show a rather different behavior of the column tips. Pole figure analy-sis reveals that the 0° coating has a preferred ⟨111⟩ orientation [23], consistent with the triangularshape of the pyramidal tips in Figure 5(a). It is noted, however, that the bounding sides of thetips are not well developed facets and in some cases the pyramids are truncated, showing a flattriangular facet normal to the growth direction. This suggests that the active growth planes maynot be at an angle to the column axis, as in the 1100°C coating, but rather parallel to the substrate,as is often the case for epitaxial growth. The pyramids might then be interpreted as a mesoscopicform of “island growth” [21], which could explain why the sides are not flat as ledge propagationwould not normally occur on them. The details of the mechanism and the underlying reasons forthis behavior remain the subject of current work.

The effects of an oblique VIA on the column texture is also different at the two tempera-tures. While no change in the growth axis is observed at 1100°C, pole figure analysis revealsthat the columns exhibit a mix of two orientations at 900°C, namely a dominant ⟨101⟩ and a sec-ondary ⟨111⟩. Comparison of Figures 5(a) and (c) also reveals that the tip facets change frompyramidal to a “rooftop” shape, showing similarities with the higher temperature tips, cf. Figure5(d), but with facets which are generally less well developed. One might hypothesize from theseobservations that the ⟨111⟩ mode of column growth is relatively unstable, perhaps because it re-lies on one dominant facet perpendicular to the growth axis. A substantially oblique VIA appearsto activate an alternate set of facets which leads to ⟨101⟩ growth. The transition, however, is notwell understood and requires further investigation.

The present TBCs also show differences in texture with previous reports in the literature.The ⟨101⟩ texture prevailing in three of the four coatings is not commonly observed, and whennoted in other studies it was concluded to be not a true fiber texture but an apparent out-of-planeorientation resulting from the growth of ⟨001⟩ textured columns at ~45° from the film plane [19].It is also evident that under some conditions changes in VIA may indeed change the crystallo-graphic axis of the columns, as suggested by some authors [16], but also that the column axismay remain invariant with changes in VIA, as indicated by others, e.g. [19]. In any event, if thecolumns do grow with well defined crystallographic axis and the column orientations relative tothe substrate may be changed arbitrarily by changes in VIA, there is no obvious reason why thecoatings should exhibit any specific out-of-plane orientation and textures reported in that frame ofreference are not particularly relevant.

Figure 6. SEM images showing details of the columns near the growth surface in the 1100°C coat-ing deposited at 45° VIA. The views correspond to the edges of fracture surfaces normal to thecoating plane. (a) View normal to the plane of incidence; and (b) view along to the plane of inci-dence in a direction approximately perpendicular to the axes of the columns.

The Role of Rotation

It is noted from Figures 5(c) and (d) that the distribution of intercolumnar porosity inducedby an oblique VIA on a non-rotating substrate is anisotropic. Figure 6(a) shows that open gapsbetween columns are clearly evident along the plane of incidence (POI, defined by the directionof vapor incidence and the normal to the substrate). In contrast, a view along to this plane andnormal to the column axes, Fig. 6(b), reveals a much lower density of intercolumnar gaps. Onewould then anticipate that the in-plane compliance of such coating might be much higher in theformer direction than in the latter. Moreover, the scale of the intercolumnar gaps in Figure 4 isprobably much larger than needed for compliance, especially for the coating deposited at 1100°C,and might be detrimental to the resistance of the coating to erosion and/or chemical attack.

The role of rotation emerges now more clearly in the context of these issues. Rotating thesubstrate along an axis perpendicular to the POI would shift the column axis back to normal tothe plane of the coating, but the sunrise-sunset effect would produce shadows on both sides ofthe column during deposition, which would lead to the formation of intercolumnar gaps narrowerthan those produced by a fixed VIA. On the other hand, rotation about a single axis may not al-leviate significantly the anisotropy of the film, as shadowing effects parallel to the rotation axiswould be less pronounced than along the POI. Minimization of this anisotropy requires obliquevapor incidence in at least two non co-planar directions, such as might be produced by simulta-neous rotation and cyclic tilting of the substrate. Substrate rotation and tilting are then importantnot so much as means to produce shadowing, which can be achieved simply by oblique vaporincidence on a stationary substrate, but primarily as a way of controlling the pattern of shadow-ing around the columns. The present findings also suggest that substrate rotation could alsomodify the selection of active facets and hence the column texture, in agreement with observa-tions reported by Shultz et al. [24]. In current technology, however, the pattern of manipulationof the substrate in the vapor plume is defined predominantly by the requirements of conformalcoverage and coating thickness distribution, with much less attention given to its potential role incontrolling the microstructure and local properties of the TBC.

Other Manifestations of Sha d owing

Closer examination of the in-tercolumnar gaps in Figure 4(b) re-veals finer scales of “columnargrowth” taking place within theseregions, as illustrated in Figure 7.On the downstream side of the gapthere is evidence of “feathering”,consisting of submicron size “cells”growing off the column side in adirection approximately parallel tothe vapor flux. This form of growthis absent on the other side of thegap, which shows branching rod-like features parallel to the columnaxis. The fine cells are a manifesta-tion of the fact that the VIA does nothave a single value but represents

Figure 7. Details of the intercolumnar gap in a coatinggrown at 1100°C with 45° VIA. The image was taken from afracture surface along the plane of incidence.

the mean of a cone of vapor directions emanating from different points in the source. The tipedge blocks the majority of the flux and produces a shadow, but there is a small fraction of vaporthat can penetrate the gap and impinge on the column side at a locally high VIA, yielding nano-scale “columns” that produce the feather-like appearance. The deposition is still line-of-sight asno growth occurs on the opposite side, which is completely shadowed by the tip edge. One mayhypothesize that the nano-scale columnar growth mode is a consequence of finer scale shadowingproduced by surface features on the side of the main column. However, futher work is needed toelucidate the nature of these features and the details of their interplay with the vapor flux.

The existence of a distribution of VIAs is likely to be also the cause for the small amountsof columnar porosity noted in Figure 3. One can readily estimate for the conditions of the presentexperiments that a flat substrate under nominally normal incidence (average VIA = 0°) actuallyhas a range of VIA from ~0±5° for a point directly above the center of the source, to ~10±5° forthe ends of the substrate (cf. Figure 2b). This appears to give rise to sufficient shadowing to in-duce the formation of porosity, even at temperatures on the order of 0.46TM.

C o n c l u d i n g R e m a r k s

The present investigation has provided new insight on how shadowing mechanisms operateto generate the intercolumnar porosity which is essential to the strain tolerant performance of EB-PVD TBCs, and on the connection between porosity and texture evolution. It has also beenshown that substantial changes in the structure of the coating may occur even within the range ofprocessing temperatures typical of industrial practice, with potentially important implications forTBC performance. These findings provide an improved foundation on which to build a betterunderstanding of the effects of substrate manipulation during deposition.

A c k n o w l e d g m e n t s

This research was sponsored by the Collaborative UC-Los Alamos Research program undergrant CULAR-9830. Additional support for S.G. Terry through the NSF Fellowship programand for J.R. Litty under DOE/AGTSR contract 98-01-SR068 are gratefully acknowledged.

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